124 CLASSICS OF MODERN SCIENCE 

 foreign materials, and that it must be purified from these admixed 

 foreign particles by agitation and filtration with various liquids. I 

 believe that there would be no hesitation in accepting this opinion, 

 if one could only demonstrate clearly by experiments that a given 

 quantity of air is capable of being completely converted into fixed or 

 other kind of air by the admixture of foreign materials ; but since 

 this has not been done, I hope I do not err if I assume as many kinds 

 of air as experiment reveals to me. For when I have collected an 

 elastic fluid, and observe concerning it that its expansive power is 

 increased by heat and diminished by cold, while it still uniformly 

 retains its elastic fluidity, but also discover in it properties and be- 

 havior different from those of common air, then I consider myself 

 justified in believing that this is a peculiar kind of air. I say that 

 air thus collected must retain its elasticity even in the greatest cold, 

 because otherwise an innumerable multitude of Varieties of air would 

 have to be assumed, since it is very probable that all substances can 

 be converted by excessive heat into a vapour resembling air. 



6. Substances which are subjected to putrefaction or to destruction 

 by means of fire diminish, and at the same time consume, a part 

 of the air; sometimes it happens that they perceptibly increase the 

 bulk of the air, and sometimes finally that they neither increase nor 

 diminish a given quantity of air — phenomena which are certainly re- 

 markable. Conjectures can here determine nothing with certainty, 

 at least they can only bring small satisfaction to a chemical philoso- 

 pher, who must have his proofs in his hands. Who does not see 

 the necessity of making experiments in this case, in order to obtain 

 light concerning this secret of nature ? 



7. General properties of ordinary air. 



(i.) Fire must burn for a certain time in a given quantity of air. 

 (2.) If, so far as can be seen, this fire does not produce during com- 

 bustion any fluid resembling air, then, after the fire has gone out of 

 itself, the quantity of air must be diminished between a third and a 

 fourth part. (3.) It must not unite with common water. (4.) All 

 kinds of animals must live for a certain time in a confined quantity 

 of air. (5.) Seeds, as for example peas, in a given quantity of 

 similarly confined air, must strike roots and attain a certain height 

 with the aid of some water and of a moderate heat. 



Consequently, when I have a fluid resembling air in its external 



